Auto merge of #104977 - RalfJung:ptr-from-ref, r=dtolnay

add ptr::from_{ref,mut}

We have methods to avoid almost all `as` casts around raw pointer handling, except for the initial cast from reference to raw pointer. These new methods close that gap.

(I also moved `null_mut` next to `null` to keep the file consistently organized.)

r? libs-api

Tracking issue: rust-lang/rust#106116